The BDB lexicon simply states it as being a ladder, the HALOT defines it as an "ascending series of stones, staircase (more probable than ladder)", which would provide you with a link to the structure of a ziggurat.

The problem that we have with knowing with certainty if this is the case, is that neither of these lexical entries give root/loan word data, or any other background material as to why they came to that definition, which is something that they normally do. I only have the abridged study edition of the HALOT at this time, so it is possible that the full version includes this supporting information (referencing scholarly journals and whatnot).

The word "sulam" is only found here in Scripture, so comparative studies within the canon wouldn't be of any help.

Anyone here with the unabridged HALOT, or the newer, multivolume set of the Dictionary of Classical Hebrew want to take a stab at it (although I don't think they've quite made it to the letter "samek" yet)?

__________________
Adam J. Myer
Pulpit Supply for PCA/OPC/URCNA congregations in the spiritual wasteland known as the Pacific NW
Evergreen Presbyterian Church (PCA)
Hinterlands of Oregon

Soli Deo Gloria